1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more particularly to a system in which a linear-type solenoid valve is utilized for controlling the supply of the secondary air.
2. Description of Background Information
Air intake side secondary air supply systems are known as systems in which oxygen concentration in the exhaust gas of the engine is detected and an air-fuel ratio of the mixture to be supplied to the engine is feedback controlled in response to a result of the detection of the oxygen concentration for the purification of the exhaust gas and the improvement of the fuel economy. As an example of the air intake side secondary air supply system, Japanese Patent Publication No. 55-119941 discloses a system which uses a linear-type solenoid valve whose opening degree is varied with the magnitude of a current supplied to its solenoid. The linear-type solenoid valve is disposed in an air supply passage leading to a carburetor of the engine downstream of the throttle valve and a sectional area of the air supply passage is continuously varied by the linear-type solenoid valve in response to a result of the detection of the oxygen concentration.
However, in this air intake side secondary air supply system, there was a drawback that the magnitude of the current supplied to the solenoid of the linear-type solenoid valve is not responsive to the load of the engine. Therefore, it was sometimes experienced that the system fails to adjust the air/fuel ratio rapidly toward an appropriate value in response to changes in the driving state. In short, the system was not responsive enough to the change in the driving state.